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K&n Velocity Stacks


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#1 Minitrio

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Posted 28 December 2015 - 02:20 PM

Hi All,

 

I have seen K&N velocity stacks in a customer recommendations section on a well known parts supplier. I already have a K&N cone fitted to my 98 998 with stage one kit.

 

Could someone explain why I need two and how I fit them, plus are they worth fitting?

 

Thanks



#2 Anthony30

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Posted 28 December 2015 - 02:26 PM

Two would be for twin carbs. A velocity stack basically help get more air into the carb, pretty interesting and simple really. Mini Spares do a kit from MED, with all the required bits. :highfive:



#3 Cooperman

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Posted 28 December 2015 - 04:22 PM

You probably won't notice any difference by fitting a stub-stack. They come into their own at very high flow rates which equals very high revs.

The idea of them is to smooth out the flow into the carb by, effectively, making the carb entry throat longer thus allowing the flow to become stable before it gets to the butterfly valve and carb needle. The higher the speed of the inlet air the longer the throat it needs to 'settle down' and become smooth.

At lower revs it is only necessary to put a generous radius on the carb inlet mouth, say 4 mm radius and make sure the surface of the carb mouth is smooth.

On some full race engines a very long inlet stack is fitted due to the ultra-high revs being used and the consequently much higher inlet volume of air.



#4 Minitrio

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Posted 28 December 2015 - 05:09 PM

Thanks guys, probably not worth investing in until i start hitting stage 3 tuning or have a larger capacity engine fitted....


Edited by Minitrio, 28 December 2015 - 05:09 PM.


#5 timmy850

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Posted 28 December 2015 - 08:36 PM

You can get them pretty cheap on ebay

http://www.ebay.co.u...2-/181536078176



#6 nicklouse

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Posted 28 December 2015 - 09:30 PM

You probably won't notice any difference by fitting a stub-stack. They come into their own at very high flow rates which equals very high revs.
The idea of them is to smooth out the flow into the carb by, effectively, making the carb entry throat longer thus allowing the flow to become stable before it gets to the butterfly valve and carb needle. The higher the speed of the inlet air the longer the throat it needs to 'settle down' and become smooth.
At lower revs it is only necessary to put a generous radius on the carb inlet mouth, say 4 mm radius and make sure the surface of the carb mouth is smooth.
On some full race engines a very long inlet stack is fitted due to the ultra-high revs being used and the consequently much higher inlet volume of air.


So in other words fit stub stacks as they make a difference on any build.

No point radius in a carb opening if the filter base plate covers it.

#7 Cooperman

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Posted 28 December 2015 - 09:48 PM

 

You probably won't notice any difference by fitting a stub-stack. They come into their own at very high flow rates which equals very high revs.
The idea of them is to smooth out the flow into the carb by, effectively, making the carb entry throat longer thus allowing the flow to become stable before it gets to the butterfly valve and carb needle. The higher the speed of the inlet air the longer the throat it needs to 'settle down' and become smooth.
At lower revs it is only necessary to put a generous radius on the carb inlet mouth, say 4 mm radius and make sure the surface of the carb mouth is smooth.
On some full race engines a very long inlet stack is fitted due to the ultra-high revs being used and the consequently much higher inlet volume of air.


So in other words fit stub stacks as they make a difference on any build.

No point radius in a carb opening if the filter base plate covers it.

 

They will make no noticeable difference at low air flow requirements. In fact, I doubt a rolling road would show any torque increase. With a so-called 'Stage 1' kit the airflow requirements are moderate. With stub-stacks the joint at the mounting must be very carefully matched with carb, base plate a d stub stack, together with the gaskets, all being fitted to give no 'step' anywhere around the diameter. 

 

Obviously the hole in the filter must be rebated to clear the radius or it won't work at all. I didn't think I actually needed to STFO.



#8 86mayfair

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Posted 28 December 2015 - 10:15 PM

As I was taught on my degree, longer trumpets aid low RPM torque, and shorter trumpets aid high RPM power. This makes sense when you consider pulse tuning



#9 Alex_B

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Posted 28 December 2015 - 10:21 PM

As I was taught on my degree, longer trumpets aid low RPM torque, and shorter trumpets aid high RPM power. This makes sense when you consider pulse tuning

Also on a mini an exponentially increasing shaped velocity stack can help in reducing the pulse that occurs as it can create conflict between cylinders due to the siamese port situation. 



#10 86mayfair

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Posted 28 December 2015 - 10:41 PM

 

As I was taught on my degree, longer trumpets aid low RPM torque, and shorter trumpets aid high RPM power. This makes sense when you consider pulse tuning

Also on a mini an exponentially increasing shaped velocity stack can help in reducing the pulse that occurs as it can create conflict between cylinders due to the siamese port situation. 

 

Ah yes I hadn't even considered the siamese ports! I suppose that does explain why the stacks on minis tend to open more 



#11 Earwax

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Posted 29 December 2015 - 07:49 AM

Has anyone got any data ( evidence) of any of the sub stacks' effectiveness?

 

I am particularly interested in fully rolled vs opened out ( as per original weber Stacks)  and also any back to back data on length differences.

 

I am with Cooperman in not noticing any marked differences at the lead into the carbs ( providing no obvious blocks/ courseness, lips etc), however the length of the inlet manifold AND the venturi transitions through the carb and port DO make a difference.. and this is where i do see evidence.. eg   a 5 inch inlet VS 3.5 can see differences in torque curves.

 

To the OP.... not saying don't investigate but stay away from the cheap cast ones, the ones i have seen are ******* ridden and would probably increase turbulence 



#12 1968andyf

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Posted 29 December 2015 - 08:33 AM

Vizard has a section on it in the yellow book

#13 Midas Mk1

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Posted 29 December 2015 - 09:23 AM

To those who say you dont notice a difference, try the MED one. Works a treat for me :)






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